1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode electronic power converters.
2. Description of Related Art
FIG. 1 illustrates a ZVS coupled inductor buck converter employing a simple capacitor coupled floating drive circuit for driving a high side power mosfet switch, MAUX. The floating drive circuit is powered by a floating bootstrap capacitor, CBOOT, that is charged through DBOOT during the on time of the main switch, MMAIN. When the PWM drive signal for the main switch MMAIN goes low the main switch turns off and the same PWM drive signal that turns off the main switch is transmitted via CAUX to the input of the inverting gate driver IC, UAUX, forcing the output of the inverting gate driver IC, UAUX, to go high, turning on the auxiliary switch, MAUX. During this transition the source voltage of the auxiliary switch MAUX is rising quickly, which forces the input to UAUX low relative to the floating ground reference terminal of UAUX, due to the coupling capacitor CAUX which, absent any change in the PWM drive signal, tries to hold the input terminal of UAUX fixed with respect to primary circuit ground. The input signal to UAUX is held low during the switching transition as a result of the voltage rise of the source of MAUX during the switching transition after UAUX initially changes state as a result of the drive signal provided to turn off the main switch. The clamp diodes at the input to UAUX prevent the input voltage to UAUX from exceeding the supply voltage rails of UAUX and the resistor RAUX limits the current in the clamp diodes to a current less than their maximum current rating during the switching transitions.
When the PWM drive signal to the main switch goes high, this same PWM gate drive signal is transmitted to the input of UAUX by CAUX, which results in the output of UAUX going low which turns off MAUX at the same instant that MMAIN is turned on. During the subsequent switching transition the source voltage of UAUX falls rapidly, which forces the input to UAUX high, relative to the floating ground reference terminal of UAUX, due to the coupling capacitor, CAUX, which, absent any changes in the PWM drive signal, tries to hold the input to UAUX fixed with respect to the primary circuit ground. A high input signal to UAUX results from the voltage fall of the source of MAUX during the switching transition after UAUX initially changes state as a result of the drive signal provided to turn on the main switch. The capacitor coupled floating drive circuit of FIG. 1 works well when the source voltage of MAUX does not change during the on time of MAUX. A drop in voltage at the source of MAUX during the on time of MAUX can turn off MAUX, which would be an undesirable result.
FIG. 1 illustrates a ZVS coupled inductor buck converter employing a simple floating drive circuit based on capacitor coupling to an inverting driver integrated circuit (IC), UAUX. This floating drive circuit works well where the input source voltage is invariant, but the circuit can change state if there are voltage variations of the input source during the time that the switch MAUX is turned on causing erratic operation, power losses, and, in some cases, component failure. What is needed is a similar simple floating drive circuit that is tolerant of voltage variations of the input source.